Never install mods in the Steam directory (ex: C:\Program Files (x86)\Steam\SteamApps\common\ImperatorRome\mod\), as the mod would not be properly loaded.

Check in the mod description the game version compatible with the version of the mod. If needed, rollback to an older patch, using the "Steam Beta" tab.

In case of manual installation, the steps are usually the following (refer to specific mod instructions, if any):

Remove any existing installation of the mod (mod folder and .mod file). This is very important, as the game loads all files present in mod folder, so old mod files that are no longer present or were renamed in a new version of the mod would conflict.